How to measure CMRR?

nickplay · November 2017

Hello,

I have tried to measure the OpenBCI h/w specification such as SNR, CMRR et al.

Then I found the Tech Overview page (http://openbci.com/index.php/techoverview) and i tried to follow the CMRR test.

However there is no detail instruction of CMRR test.

Could anyone know how to measure such specification of OpenBCI board?

wjcroft · November 2017

Nick, hi.

There is an old CMRR related thread here,

http://openbci.com/forum/index.php?p=/discussion/154/front-end-filtering-esd-cmrr-rfi

With Joel's comment at,

http://openbci.com/forum/index.php?p=/discussion/comment/977/#Comment_977

Various tutorials on the web,

https://www.dataq.com/blog/data-acquisition/calculate-common-mode-rejection-ratio/

https://www.google.com/search?q=measuring+cmrr

nickplay · December 2017

wjcroft, hi.

I read the articles what you recommended. Then i tried to test using the parameters from the link: https://docs.google.com/spreadsheets/d/1IHy0lOcYJDAzqx9n2essxVWIHo0QjdH9r8cdquoV4pY/edit#gid=0

From the Sheet1, i can't understand what "output Amplitude" value means. How can i get the value from the data sheet such as sheet 2~10?

Thank you.

wjcroft · December 2017

On that spreadsheet, the output amplitudes are in volts. Thus .000001 is 1 microvolt.

nickplay · December 2017

I mean, how can i get the number 1uV using the data sheet2?

wjcroft · December 2017

We would have to ask Joel @biomurph how he derived the summary info in sheet1.

My guess-estimate, is that it looks like 1.1v input signal was a peak to peak measurement? Then looking at the output values in each sheet*, some type of averaging function was applied that estimated the maximum mean plus and minus deviations (in microvolts shown on the graph) of the output signal. Then subtracted those two max min values to get the output voltage, peak to peak.

Just guessing here. Joel, please jump in.

biomurph · December 2017

@nickplay

That work was done a long time ago, but I do remember that my measurement of CMRR involved inserting a signal of a known amplitude/RMS power level into both inputs of one of the differential channels, and then measuring the output amplitude/RMS power level that the ADS gave me. From my understanding, the ratio is between the two, input and output.

I generated a signal using a sine generator (Arduino DUE) and I reduced the amplitude with a transformer.

How to measure CMRR?

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